The Challenges of Welding Reactive Metals
Reactive metals such as zirconium, titanium, and beryllium, along with refractory metals like tantalum and columbium (niobium), are increasingly being used in aerospace, automotive, medical, and military industries due to their durability, low density, biocompatibility, and high corrosion resistance. However, these materials are expensive, and welding procedures must be carefully developed and strictly applied to avoid costly waste, rework, or risk of service failure.
Welding reactive and refractory metals presents unique challenges due to their strong affinity for oxygen at high temperatures. This can lead to oxidation, which discolours the weld and significantly reduces its corrosion resistance and mechanical strength. While fusion joining techniques for these metals have evolved over the years, thorough pre-cleaning and protection against oxidation during welding remain critical.
Cleanliness:
Welding reactive metals, particularly during repair work on equipment in service, is challenging due to the contamination from process scale, dirt, and other debris. Outdoor welding, where environmental conditions such as high humidity and wind can interfere, further complicates the process. Poor surface preparation can lead to weld contamination, resulting in premature failure and the need for repeat repairs. Surfaces must be meticulously cleaned, degreased, and dried in controlled factory environments before welding to prevent contamination.
Protection Against Oxidation:
When exposed to oxygen during welding, reactive metals form stable oxides, leading to discoloured welds and reduced mechanical properties. For titanium and similar alloys, it is essential to maintain oxygen levels below 20 ppm to avoid oxidation. Standard gas tungsten arc welding (GTAW) torches do not provide sufficient coverage, necessitating specialised purging equipment.
Solutions for Welding Reactive Metals
Trailing Shields®:
To provide adequate protection against oxidation, Argweld® Trailing Shields® have been developed to offer additional inert gas shielding during welding. These shields protect the fusion zone and the hot adjacent metal, ensuring that the entire weld area is covered. Argweld® Trailing Shields® are available for various applications, including curved shapes like pipes and tubes, flat sheets, and rectangular tanks. These shields can be used with GTAW (TIG) and PAW (plasma) welding torches and are designed to supplement the essential torch's shielding capabilities.
Pipe and Tube Purging:
Effective root protection is crucial for welding pipes and tubes. Early attempts at creating purge dams using makeshift materials such as paper, card, or polystyrene were often unsuccessful, leading to inadequate sealing and contamination. Argweld® systems, including QuickPurge® and PurgElite®, have been developed to provide reliable and efficient purging solutions. These systems are designed for easy insertion, positive sealing, and controlled gas flow to protect the weld root from oxygen fully.
Flexible Enclosures®:
For the highest level of protection, welding can be conducted in a vacuum or within metal enclosures filled with inert gas. However, these methods are expensive and typically justified only in high-volume production. A more cost-effective alternative is the use of Flexible Welding Enclosures®. These enclosures, made from ultra-violet stabilised engineering polymers, provide a controlled environment for welding reactive metals. They have leak-tight zips, glove ports, and entry points for welding torches and other equipment.
Residual Oxygen Measurement:
Accurate oxygen monitoring is essential for ensuring the welding environment remains contaminant-free. PurgEye® Weld Purge Monitors®, such as the PurgEye® 300 Plus and PurgEye® 500 Desk, have been developed to meet the stringent requirements of welding reactive and refractory alloys. These instruments can measure oxygen levels to 10 ppm, providing the precision needed to maintain weld quality.
Conclusion
Welding reactive metals requires careful attention to cleanliness, protection from oxidation, and precise control of the welding environment. Tools like Argweld® Trailing Shields®, QuickPurge® and PurgElite® purging systems, Flexible Welding Enclosures®, and PurgEye® Weld Purge Monitors® are essential for overcoming the challenges associated with these materials. By employing these advanced solutions, fabricators can ensure the integrity of their welds, avoid costly rework, and meet the stringent demands of high-performance industries.
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